Use of immersion tin and tin alloys as a bonding medium for multilayer circuits

ABSTRACT

A novel immersion tin composition is disclosed containing both thiourea compounds and urea compounds. A method for improving the adhesion of printed circuit boards to one another in a multilayer board and for minimizing or eliminating smear in a multilayer board is also disclosed comprising coating the metal layers of the individual circuit boards with an immersion tin coating prior to laminating them to form a multilayer board.

This is a divisional of copending application Ser. No. 770,842, filed onAug. 29, 1985 now U.S. Pat. No. 4,715,884.

DESCRIPTION

1. Technical Field

The field of the invention is printed circuit boards and a method forimproving the manufacture of such boards by employing a tin immersioncoating. Improved adhesion of laminae in a multilayer board, betterthrough hole plating and substantial elimination of smear are obtainedby employing an immersion tin coating. A novel immersion tin coating isalso disclosed.

2. Prior Art

Methods for improving the adhesion of metallic layers to non-conductingsurfaces of printed circuit boards in a multilayer board are disclosedby Luce, et al., in U.S. Pat. No. 3,585,010; Berdan, et al., U.S. Pat.No. 4,260,449 and Torday, et al., U.S. Pat. No. 4,469,567. Luce, et al.,disclose the production of circuit board laminates employing a copperfoil conductor treated by electrodepositing a thin metal layer on it.The metal consists of indium, zinc, tin, nickel, cobalt, brass, orbronze. These metals optionally may be deposited on the surface of thecopper by vapor deposition.

Immersion tin coatings are disclosed by Orio, et al., British Pat. No.2,072,709; Shipley, et al., U.S. Pat. No. 3,303,029 and the variousreferences cited in Orio, et al., and Shipley, et al.

SUMMARY OF THE INVENTION

The present invention relates to a novel immersion tin compositioncomprising:

(a) a thiourea compound

(b) a urea compound

(c) a chelating agent

(d) a reducing agent

(e) an acid and

(f) a tin salt

The invention also relates to a process for improving the adhesion of ametal layer of a printed circuit board to the non-conductive surface ofa circuit board comprising coating the metal layer with an immersion tincomposition especially the composition of the invention. By eliminatingthe black copper oxide layers of the prior art as an adhesion promotinglayer, better through hole plating is realized.

By coating the metal layers of a multi layer circuit board with animmersion tin composition prior to drilling through holes in the board,smear in the holes is eliminated or substantially eliminated therebyminimizing or eliminating the etch-back treatment of the through-hole.Although the inventors do not wish to be limited by any theory, it isbelieved that the tin acts as a lubricant and to dissipate heat duringthe drilling of the holes. The composition of the invention isespecially useful for this purpose.

DETAILED DESCRIPTION

The present invention relates to a novel immersion tin composition andits use in the manufacture of printed circuit boards. Printed circuitboards (PCB's) comprise a non-conducting or dielectric base made up of afibrous material such as glass fibers, paper and the like, impregnatedwith a resinous material such as an epoxy or phenolic resin. The boardis clad with a metal conductive layer such as copper on either one orboth surfaces. Multilayer boards (MLB's) comprise several circuit boardslaminated to one another by means of an adhesive. In addition to rigidboards (as described above), flexible boards can be produced employingthermoplastic dielectric layers such as fluorocarbon polymers, Nylonpolymers, polyimides, polyparabanic acids, and polyesters. Production ofthese types of printed circuit boards are described in Printed CircuitsHandbook, Second Edition, edited by C. F. Coombs, Jr. McGraw-Hill, 1979,which is incorporated herein by reference. Laminated combinations offlexible and rigid boards are also finding utility in some more recentapplications for MLB's. The present invention applies to all of thesestructures.

In the manufacture of PCB's, a metal conductive foil such as copper isbonded to the circuit board, although for the purposes of the presentinvention any metal applied to a non-conductive dielectriccircuit boardas a foil or by electro deposition, electroles deposition.

Prior to laminating, the metal surface is treated in an art known mannerto produce electrically-conductive lines (circuits) for the transfer ofcurrent between components of an electrical circuit, the componentscomprising by way of example, diodes, transistors, resistors,capacitors, and the like. The circuits may be formed either by apositive or a negative working photo resist, silkscreen resist, or handpainted resist process followed by etching and in some instances,electrodeposition of a metal or metals, all of which is known in theart.

The circuits of the inner layers of an MLB are formed by a print andetch process in which a positive working photo resist is applied to themetal layer, a positive circuit image projected onto the resist,followed by a developing process to leave a positive resist pattern. Thepositive resist pattern can also be formed by a negative working photoresist through which a negative circuit image is projected. The board isthen etched after which the positive resist is removed. The elements ofthe circuit obtained do not have to be electrically connected to oneanother. These circuits elements that are not connected are placed onthe board for subsequent connection to other boards in the MLB array.

In MLB's, the circuit of one board is connected to the circuit of one ormore of the other boards in the multilayers. This is achieved by formingpads or circular areas of metal at a point or points on the conductiveline or lines of the board. The pads may also be isolated from theconductive lines. The other board or boards that are to be connected aresimilarly provided with pads and in the laminating process the pads ofthe different boards are aligned over one another.

In forming MLB's by laminating, a prepreg is inserted between thesurfaces of the boards that are to be laminated after which the multiplelayers are further treated by applying heat and pressure to them. Theprepreg generally comprises a woven or non-woven layer or layers offibers such as glass, cellulose (e.g. paper), and the like, glass beingpreferred. The prepreg also is impregnated with a so-called "B-stage"resin such as an epoxy resin that has been partially cured. Art knownequivalents of epoxy resins are also employed as adhesives such asacrylic resins (used with polyimide circuit boards) or polyester resins.

The MLB is then pressed and cured with the pads of the circuits of thedifferent boards that are to be connected aligned over one another.After curing, the pads of the MLB's are drilled. The diameter of thedrill is considerably less than the diameter of the pad, the ratio ofdiameters between the pad and the drill being 2:1 or greater so that theoverall structure comprises at a minimum a pad from one board alignedover a pad from another board with a hole passing through them. Sincethe hole in cross-section ideally presents a surface of alternatinglayers of the pads of the individual PCB's separated by thenon-conductive base, an electrically conductive element has to beemployed in the hole to form an electrical connection between the pads.This is done by a process known in the art as through hole plating(PTH).

PTH processes are also employed for connecting two metal conductivesurfaces having a single non-conductive or dielectric board interposedbetween them for the formation of a PCB. Boards of this type and theformation of through holes in such boards are also within the scope ofthe present invention and are intended to be included within the broaddefinition of the MLB's as that term is used throughout thespecification.

Electroless copper is employed as a PTH plating material. Standardelectroless copper plating solutions known in the art are used for thispurpose. In order to promote the deposition of electroless copper on anon-conductive surface, the non-conductive surface is treated with astannous chloride sensitizer solution followed by a super sensitizersolution of di-valent palladium chloride. The stannous chloride isoxidized to stannic chloride and the palladium chloride reduced topalladium metal. A preferred method is to employ an activator comprisingcolloidal palladium containing stannic tin. Stannic tin forms aprotective colloid around the metallic palladium, and the solutionimplants a precious metal site on the non-conductive surface for thepurpose of initiating the deposition of the copper by chemicalreduction. A postactivator is then employed, generally an acid, tosolubilize the protective colloid and expose the precious metal, i.e.palladium. The subsequently applied electroless copper coating solutioncontains a reducing agent such as formaldehyde, and when in the presenceof palladium reduces the cupric ions in the solution to copper metal.The copper metal plates out on the surface of the through hole, makingelectrical contact with the walls of the metal pads through which thehole is drilled. The electroless copper may have subsequent metalcoatings applied to it by electrolytic means.

Because the outer conductive metal layer of an MLB is formed as part ofthe PTH process, a print and etch procedure cannot be employed forforming circuits on this layer. The etching step would remove theplating applied in the through hole and would be self-defeating. Forthis reason, after the PTH process is completed, a negative circuitpattern is formed on the surface of the MLB by means of a negativeworking photo resist, a positive circuit image projected onto the resistfollowed by developing. This leaves a negative photo resist circuitpattern on the board with bare metal exposed corresponding to the areaof the circuit. The negative resist pattern can also be obtained by apositive working photo resist through which a negative circuit image isprojected. The bare metal corresponding to the circuit is then coated byelectrolytic means with a second metal that acts as an etch-resist. Tinlead alloys are the most common material used in this respect. The etchresist coating also coats the metal surface applied to the through holesin the PTH process. The photo resist is then removed and the board isetched resulting in the formation of a circuit on the outer layer of theMLB having a tin lead coating on the circuit as well as on the walls ofthe through hole.

One of the difficulties encountered in the prior art manufacture ofMLB's is the low bonding strength sometimes obtained between the prepregand the metal surface of a circuit board in the array of boards thatmake up the MLB. These problems are especially prevalent when the metallayer comprises copper. The bonding between the copper clad board andthe prepreg was improved by etching the copper with amonium persulfate;however, only slightly improved bonding strengths were obtained. Someimproved adhesion over the etching process was obtained by pumicespraying to micro-etch the copper surface or provide cavities in thecopper surface to which the resin such as the epoxy resin of the prepregcould be bonded to. Other mechanical processes such as brushingsimilarly provided a mechanical alteration of the surface of the copperto improve the bond strength.

Even better bond strengths between the copper layer and the prepreg wereobtained with a so-called black oxide coating in which the coppersurface of a PCB was treated with a mixture of sodium hypochlorite andsodium hydroxide to produce a black copper oxide. Although better bondstrengths were obtained, one of the difficulties with using this methodwas that the composition of the sodium hypochlorite/sodium hydroxidesolution changed over a period of time and erratic results were obtainedsuch as varying thicknesses of the black oxide. The formation of theblack oxide coating was time and temperature dependent and as a resultthe bath employed for the development of the coating had to be closelymonitored. Additionally, prior to the formation of the black oxidecoating, the copper surface had to be prepared by brushing, rinsing, andetching either with amonium persulfate or a sulfuric acid-hydrogenperoxide mixture.

It was subsequently discovered that by lowering the sodiumhypochlorite/sodium hydroxide bath temperature from 90°-95° C. to60°-65° C. some economies could be realized in that less heat had to besupplied to the bath, but more importantly the process was easier tocontrol. By employing temperatures of from about 60°-65° C. a bronze orbrown colored copper oxide layer was produced on the surface of thecopper and slightly better bond strengths were obtained.

It was essential to control the black oxide formation on the surface ofthe copper since if the oxide was excessively thick, it would shearreadily thereby destroying the bond between the copper metal and theprepreg. Additionally at the proper thickness, the black copper oxidewas not sufficiently flexible so that it could not be used in a flexibleMLB. Even though the brown or bronze colored copper oxides obtained atthe 60°-65° C. bath temperatures were easier to control, these coatingswere not without their difficulties. Coating of this type were notreadily accepted by the purchasers of MLB's since they were mottled oruneven in color and gave the appearance that the coating was notuniform. This mottled appearance or "brown spotting" has been describedas affecting the dielectric properties of the resin, and consequently,the overall performance of the printed circuit (Cf. Berdan, et al., U.S.Pat. No. 4,260,449) as well as a lowering of the peel strength of thecopper foil (i.e. bonding strength) to the prepreg (Cf. Torday, et al.,supra, column 1, lines 27-35).

Torday, et al. (supra) and Luce, et al. (supra), describe methods forovercoming some of the difficulties encountered with the bonding of thecopper metal to the prepreg by the electrodeposition or vapor depositionof various alloys or metals to the surface of the copper prior tobonding. One of the difficulties with the deposition of a metal film onthe copper is that the processes must be closely controlled to assurethat proper metal film thicknesses will be obtained. Skilled operatorsand constant monitoring of the composition and the conditions ofdeposition are required in order to obtain uniform thicknesses of thesemetals on copper.

Even with the prior art processes for treating the copper metal surfaceto obtain better adhesion to the prepreg, the laminating of theindividual circuit boards to form the MLB was a time consuming process.In the laminating process, the individual PCB's assembled in an MLB areplaced in a press that is heated to 175° C. held there for about 10-15minutes to bring the MLB array up to temperature after which a pressureof 250-400 psi is applied. the MLB is held at this temperature and thispressure for approximately 45-50 minutes after which heating is stoppedand the MLB allowed to cool for about 10-15 minutes while in the press.After this cooling period, the pressure is released and the MLB isremoved from the press.

In subsequent operations after providing through holes and PTHconnectors, circuit components are placed on the MLB and soldered to thecircuits. Two types of soldering methods are employed one of whichcomprises a hand soldering process the other of which is an automaticsoldering method employing a wave soldering apparatus known in the artin which the bottom of the MLB through which leads to circuit componentsproject are contacted for about 60 seconds with a solder wave that ismaintained at a temperature of approximately 290° C. MLB's in which thevarious laminae are not properly bonded to one another at the interfacesof the copper foil and the prepreg will delaminate when exposed to thesesoldering conditions.

Smearing is another difficulty encountered in the manufacture of MLB'swhereby the drill bit employed to form the hole through the aligned padsin an MLB picks up resinous material and deposits this material on thewall of the hole. Since the wall of the hole contains alternatingresinous material layers and metal layers, the surface of the metallayers that form part of the hole wall will be coated with the resinousmaterial thereby preventing any metallic plating material applied to thesurface of the hole wall from contacting the metal layers and forming anelectrically conductive connection with it. It is believe that theresinous material such as a B-stage epoxy resin used in the prepregcomprises the principle material involved in the smearing of the hole.Smearing, therefore, renders the PTH process ineffective.

The problem of smearing is overcome by a process known in the art as"etch-back" in which a strong material acid such as sulfuric acid(optionally containing some hydrofluoric acid) is used to etch away the"smear" from the wall of the hole. Hydrofluoric acid is added to thesulfuric acid to remove any glass fibers from the circuit board thatmight be projecting into the hole. The glass fiber ends come from theglass fiber battings employed in the manufacture of the circuit boardsor prepreg and are removed since they cause imperfections in the surfaceof the metallic coating applied by the PTH. process.

One of the difficulties with the etch-back process is that it requiresvery careful control in order to prevent excessive etching of the wallof the hole. The concentration and temperature of the etching solutionhas to be carefully controlled and monitored as well as the length oftime over which the etching process is conducted. Again, because ofthese critical conditions, skilled operators are required to performthis process.

Precise control of the degree of etch-back is critical in order toobtain an acceptable metal coating on the wall of the through hole. Insome instances in the etch-back process, the etching solution willremove the resinous layer of the MLB preferentially over the metal orcopper layers in the hole. This will result, after etching, in the metallayers projecting slightly beyond the wall of the hole as defined by thesurface of the resinous material. If this metal layer projection isexcessive, the metal coating applied to the wall of the hole by the PTHprocess will be uneven and will not make proper electrical contact withthe metal layers in the hole.

The phenomenon of the metal layer being etched at a faster rate than theresinous layer in the hole is known as "reverse etch" which in someinstances presents the same type of problem encountered where the metallayer projects to far into the hole after etching.

Another difficulty encountered in the prior art PTH process is leaching,in which the black copper oxide formed on the copper foil is depositedas a residue in the through hole during the etch-back procedure.Although after the etch-back process is completed the hole is rinsed,the rinsing process is not effective to remove all of the black copperoxide deposited on the wall of the hole. This causes several problems,the first of which is that the black oxide may interfere with thepalladium catalyst deposited on the wall of the hole for promoting theelectroless deposition of copper in the PTH process. Secondly, becausethe black oxide is deposited on the wall as a non-adhering particulatecomposition, it interferes with the bonding of the electroless copper onthe surface of the wall of the hole. Thirdly, black oxide diffusesthrough the PTH coating resulting in pin holes which interfere with theintegrity of the coating by providing sites for the coating to beattacked when the MLB is subjected to etching after the PTH process iscompleted.

The black oxide layer that forms part of the surface of the wall of thethrough hole not only contributes to these problems but is also etchedaway in the etch-back process leaving a void in which etching solutioncomponents are trapped. These components are not always removed in therinsing step and as a result also interfere with the PTH metal layerapplied to the through hole.

Blow holes may also form in the PTH coating by the different thermalco-efficients of expansion of the black oxide and the glass materialemployed in the construction of the circuit boards. The blow holes occurwhen the board is submitted to extremes to temperature either during thesoldering operation or when the boards is in use. Extremes intemperature are also encountered in the through hole after electrolessplating of copper when a second metal coating is applied in the PTHprocess comprising a tin-lead layer that acts as an etch resist. Thetin-lead coating is applied electrolytically and subsequently heated orflowed in order to produce a tin-lead coating in the through hole havinggreater integrity. Pin holes and blow holes in the PTH coating can bethe cause of reduced conductivity in the PTH and are to be avoided forthis reason.

The use of immersion tin coatings for plating metal surfaces is known.This process does not employ an electric current but is based on anelectro chemical displacement reaction. The metal substrate that is tobe plated generally is more active (less noble) than the metal salt thatis disolved i the coating composition or plating solution. Copper may beplated by a tin solution even though copper is more noble than tin whenthe immersion coating composition is acidic and contains thiourea as aso-called "complexing" agent. It has been theorized that the relativeelectrode potentials of tin and copper are reversed under acidicconditions. Once the metal substrate is completely coated, it is nolonger available to displace the metal ions in the immersion coatingcomposition.

Some of the limitations of the immersion coating processes were the slowplating speeds, limited thickness of the coating, as well as theextensive degreasing, cleaning and oxide removal pre-coating steps thathad to be employed.

The immersion coatings were also porous. The problems of porosity wereattempted to be overcome by greater coating thicknesses. Additionally,high coating temperatures (about 82°-85° C.) had to be employed which insome instances caused noxious fumes to be produced from the coatingsolution and prevented the use of some low cost plastic materials ofconstruction for the plating tank.

Coating times of from 10 to 40 minutes were not uncommon for thisprocess. These times were employed in order to obtain nonporous,relatively thick films on the metal substrate. Additionally, the metalsubstrate such as a copper substrate employed in circuit boards had tobe grease free and oxide free in order to obtain relatively thick tincoatings of 80 millionths of an inch thick after about 40 minutes. (Cf.Scneble, Jr., U.S. Pat. No. 3,917,486).

It is, therefore, an object of the present invention to overcome theseand other difficulties encountered in the prior art.

A further object of the present invention is to provide a compositionfor the treatment of the surface of metal conductive layers of a circuitboard and especially a copper layer to promote the adhesion of the metalto a non-conductive surface.

It is a further object of the present invention to provide a process forimproving the bond strength between the conductive surfaces and thenon-conductive or dielectric layers of an MLB assembly.

It is also an object of the present invention to provide a compositionfor treating the metal surfaces of a circuit board to be employed in thefabrication of an MLB that will significantly reduce the time and/orlaminating temperature used in forming the MLB while maintaining orincreasing the bond strength between the layers of an MLB.

It is also an object of the present invention to provide a process fortreating the metal surfaces of a circuit board to be employed in thefabrication of an MLB that will significantly reduce the time and/orlaminating temperature used in forming the MLB while maintaining orincreasing the bond strength between the layers of an MLB.

It is the further object of the present invention to provide a novelcomposition for reducing or eliminating smearing in an MLB.

It is the further object of the present invention to provide a novelprocess for reducing or eliminating smearing in an MLB.

It is the further object of the present invention to provide a novelimmersion coating composition.

It is the further object of the present invention to provide an adhesionpromoting flexible metal coating on the metal surface of a circuit boardused in an MLB.

It is also an object of the present invention to provide a novelcomposition which may be employed in the production of metal immersioncoatings.

It is also an object of the present invention to provide an immersioncoating that may be applied to a metal surface that is oxidized orpartially oxidized.

It is a further object of the invention to provide a novel immersion tincoating composition that simultaneously cleans and coats a metalsubstrate to which it is applied.

It is the further object of the present invention to provide a novelmetal immersion composition that may be employed as a coatingcomposition at room temperatures.

It is the further object of the present invention to provide such animmersion composition that may be employed as a coating material thatwill deposit a uniform, non-porous coating from about 0.08 to about 0.17microns.

It is also an object of the invention to provide such a composition thatwill form a coating in about 30 seconds.

These and other objects have been achieved according to the presentinvention which relates to an immersion composition, especially animmersion tin composition comprising:

    ______________________________________                                                        (a) a thiourea compound                                       (1-a)           (b) a urea compound                                                           (c) a tin salt                                                                The composition may                                                           optionally contain                                                            (d) a chelating agent                                         (1-b)           (e) a reducing agent and                                                      (f) an acid                                                   ______________________________________                                    

The acid is employed to maintain an acid pH which prevents or minimizesany oxidation reactions whereby the tin salt which is preferably astannous salt could be oxidized to its higher valent form i.e., astannic salt. Preferred compositions comprise those employing components(a) through (f) as listed above and those optionally containing (1-c)the salt of another metal (as noted herein) and (1-d) a surfactant.

It has also been found that the aforesaid composition can be employed asa coating composition to improve the adhesion of a metal substrate of aPCB to a non-conductive surface of a circuit board and also eliminates,substantially eliminates or minimizes smearing when through holes aredrilled in an MLB. Other immersion tin compositions can be employed inthis aspect of the invention e.g. such as those disclosed in the U.S.Patents to Shipley, 3,303,029; Ceresa, et all, 2,891,871; Sullivan, etal., 2,369,620; and Bradley, 2,282,511.

By employing an immersion tin composition and especially the compositionof the present invention in the manufacture of an MLB, problems of pinholes and blow holes are eliminated, or substantially eliminated in thePTH coating.

The essential feature of the composition of the present invention is theuse of the combination of a thiourea compound and a urea compound in animmersion coating. It has been found that when using this combinationthat bright, uniform, non-porous thin coatings of tin or tin alloys canbe applied to a metal substrate. Tin or tin alloy coatings from about0.08 to about 0.17 microns are obtained that have good solderability andare good etch-resists because they are substantially non-porous.

The combination of the thiourea compound and urea compound allow coatingof metal substrates with substantially no pre-cleaning or minimalpre-cleaning. The composition of the invention acts as a cleaningcomposition to some degree and in many instances can be applied directlyon to a metal substrate without pre-cleaning.

By way of example the immersion tin coating of the invention, optionallycontaining a nickel salt and a surfactant was applied to a coppersurface that had a slight oil film, fingerprints and areas of copperoxide without any pre-cleaning of the copper surface or oxide removal.After about 15 seconds, a bright, well adhered tin nickel coating wasobtained on the entire copper surface.

Thus the novel tin immersion coating comprising the combination of thethiourea composition and urea composition has been found to act as acleaner and degreaser for the metal substrate over which the coating isapplied. Extensive oil and/or grease removal and a scrubbing of themetal substrate to remove metal oxides is either substantially reducedor eliminated. The coating composition will deposit a tin layer on ametal oxide surface such as copper oxide.

The immersion tin coatings of the invention have been similarly appliedto gold and aluminum metal substrates and substantially the same resultsobtained. The application of the coating therefore appears to beindependent of the reduction potentials of the metal substrates that arecoated.

The components of the composition are present in the following amountson a molar basis:

    ______________________________________                                                (a) about 10 to about 125 parts of a thiourea                                 compound                                                                      (b) about 10 to about 125 parts of a urea compound                    (2-a)   (c) about 1 to about 15 parts of a tin salt.                          ______________________________________                                    

where the composition optionally contains a chelating agent, a reducingagent and an acid, these components are present in the following amountson a molar basis:

    ______________________________________                                                (d) about 5 to about 40 parts of a chelating agent                    (2-b)   (e) about 5 to about 110 parts of a reducing agent                            (f) about 1 to about 30 parts of an acid.                             ______________________________________                                    

The composition optionally may contain the salt of a second metal in thefollowing amounts, on a molar basis:

    ______________________________________                                                (g) about 0.2 to about 55 parts of a salt of a                        (2-c)   Group VIII metal, a Group IVA metal (with the                                 exception of tin) a Group IIB metal and Group IB                              metal.                                                                ______________________________________                                    

A surfactant (2-d) may also be added to the composition in an amount asindicated herein.

In one aspect of the invention a composition is provided comprising;

    ______________________________________                                                    Thiourea      10-100  g/l                                                     Urea          5-80    g/l                                                     Tartaric acid 5-80    g/l                                         (3-a)       Glucose       5-80    g/l                                                     H.sub.2 SO.sub.4 (36%)                                                                      2-60    ml/l                                                    SnSO.sub.4    1-30    g/l                                         ______________________________________                                    

Similarly where a combination of tin and another metal is to be coatedsuch as nickel the composition will have the following components addedto it:

    ______________________________________                                                    Triton X-100  0.5-10  ml/l                                        (3-b)       NiSO.sub.4    5-80    g/l.                                        ______________________________________                                    

In one embodiment, a coating composition is prepared comprising thefollowing:

    ______________________________________                                                 Thiourea            60 g/l                                                    Urea                40 g/l                                                    Tartaric Acid       40 g/l                                                    Glucose             40 g/l                                                    H.sub.2 SO.sub.4 (36%)                                                                            30 ml/l                                          (4)      SnSO.sub.4          20 g/1                                                    Triton X-100 (1) (optional)                                                                        5 ml/1                                                   NiSO.sub.4 (optional)                                                                             40 g/l                                                    Water               balance                                          ______________________________________                                         (1) nonionic polyoxyethylene nonyl phenol surfactant.                    

Coating compositions containing about 60 g/l thiourea; about 40 g/lurea; about 40 g/l NiSO₄ and the balance water have been prepared andcan form immersion tin coatings on a copper substrate.

The various equivalents as set forth herein for the foregoing componentsof formulas (1-a) through (1-d); (2-a) through (2-d); (3-a) and (3-b);and as known in the art may be substituted for such components on anequimolar basis.

Various combinations of tin and other metals may be used in thecomposition as soluble salts e.g. water soluble salts based on mineralacid or organic acid salts of these metals. The metals that may be usedwith tin include the soluble e.g. water soluble salts of a metal takenfrom Group VIII, Group IB, Group IIB and Group IVA (with the exceptionof tin) of The Periodic Table of the Elements and mixtures thereof.Thus, in addition to tin nickel, tin lead or tin mercury coatings, maybe deposited by means of the immersion composition of the invention.

Additionally, other immerision compositions, useful for plating metalsurfaces are prepared by substituting other soluble, e.g., water solublemetal salts for tin of formulas (1-a) through (1-d); (2-a) through(2-d); (3-a) and (3-b); and (4), these other metal salts comprisingthose based on the metals of Group IVA; VB; VIB; VIIB; IB; IIB and IIIAof the Periodic Table of the Elements; the group IVA, VIII, IB, IIB andIIIA metals being preferred; and the Group IVA, VIII and IB metals beingespecially preferred. As referred to in the immediately precedingdescription, the Group IVA metals are intended to include the Group IVAmetals with the exception of tin. The anions of these metal salts arethe same as those defined herein for the tin salts. Additionally, themetal salts as defined above and herein are preferably employed in theirlowest oxidation states e.g.,

Sn(2); Ni(2); Hg(1); Au(1) and the like. Various mixtures of these othermetal salts may also be employed. Salts of lead, mercury, silver,indium, gold and palladium are especially suitable.

The tin coating compositions of the invention are useful as an etchresist in lieu of electrolytic tin lead etch resists used in themanufacture of an MLB as described herein.

The foregoing tin immersion coating provides many advantages over thecoatings employed in the prior art especially the tin immersion coatingsof the prior art. Thinner pore free coatings are obtained i.e. coatingsof about 0.08 to about 0.175 microns compared to the prior art in which1-2 micron coatings (40-80 millionths of an inch) are utilized to obtainpre free coatings.

The composition of the present invention will produce coatings of fromabout 0.001 to about 0.175 microns and especially from about 0.05 toabout 0.175 microns. The coating times will vary from any where betweenabout 2 seconds up to 5 minutes (especially 5 seconds to about 120seconds), suitable coatings (about 0.065 microns) being obtained atabout 30 seconds. The coating temperatures may vary any where from about0° to about 30° C., especially from about 15° to about 25° C. althoughhigher temperatures may also be employed.

Excellent adhesion of the inner layers of an MLB are obtained when themetal surfaces of the inner layers are coated with the immersion tincoating of the present invention prior to laminating. No delamination ofan MLB prepared in such a manner was noted after exposure of the MLB toa temperature of 290° C. for 10-60 seconds.

In addition to eliminating or substantially eleminating smearing in thethrough hole of an MLB, by acting as a lubricant for the drill bit aswell as a heat dissipating material, the immersion tin coating of theinvention also reduces the bonding times and/or temperatures employed inmaking the MLB. Typically, by coating the inner layers of an MLB priorto laminating with the tin immersion coating, the preheat step in theMLB laminating process may be eliminated and the laminating time reducedto about 5 to about 30 minutes from the standard 45 minute cycleordinarily required for this step. Pin holes and blow holes in PTHcoatings are also substantially eliminated or minimized when employingthe coating to promote adhesion of a prepreg to a metal layer in an MLB.

Because the immersion tin coating layer is used in lieu of a blackcopper oxide layer to promote adhesion to the prepreg, excessive etchingof the adhesion-promoting layer is avoided with all of the problemsassociated with it in the etch-back and PTH processes. The immersion tinlayer is substantially unaffected in an etch-back process as compared tothe black oxide layer.

Although various other metals may be deposited with the tin in the novelimmersion coating, it is preferred to use a nickel salt such as nickelsulfate in the coating composition when the coating layer is to beplated with another metal. The other Group VIII metals may also beemployed in this aspect of the invention as well as the Group IB metalsIIB metals and the Group IVA metals (with the exception of tin) andmixtures thereof.

The thiourea compounds and urea compounds of the present invention maybe represented by the formula:

    (C)(X)(N).sub.2 (R.sup.1).sub.a (R.sup.2).sub.a (R.sup.3).sub.a (R.sup.4).sub.a

where

    ______________________________________                                        R.sup.1 = (1) H                                                               (2) alkyl or alkenyl having up to about 6                                     carbon atoms                                                                  (3) aryl, aralkyl, or alkaryl having up to                                    about 12 carbon atoms                                                         (4) cycloalkyl, cycloalkylalkyl, or                                           alkylcycloalkyl having up to about 12 carbon atoms                            (5) R.sup.5 (CO) where R.sup.5 may be the same as                             radicals (2) through (4) as defined above for R.sup.1                         (6) CH.sub.2 OH                                                               (7) (C)(X)(N).sub.2 (R.sup.1).sub.a (R.sup.2).sub.a (CO).sub.2                (8) (C)(X)(N).sub.2 R.sup.1 R.sup.2 R.sup.3                                   (9) (C)(X)(N) R.sup.1 R.sup.2                                                 (10) (C)(X)(N).sub.2 R.sup.1 R.sup.2 (CO)                                     ______________________________________                                    

R², R³ and R⁴ may be the same as radicals (1) through (6) as definedabove for R¹.

a=1 or 0

For the thiourea compounds and their analogs employed according to thepresent invention X=S, Se or Te; X preferably is S.

For the urea compounds employed according to the present invention,X=oxygen.

Mineral acid salts of the thiourea and urea compounds may also be used.

The thiourea compound of the present invention comprises either thioureaor the various art known derivatives, homologs, or analogs thereof.Compounds that may be employed in this respect comprise2,4-dithiobiuret; 2,4,6-trithiotriuret; alkoxy ethers of isothiourea;thiocyanuric acid (trimer of thiourea); thioammelide (trimer ofthiourea); monalkyl or dialkyl thiourea, where the alkyl group comprisesa lower alkyl group, having up to about four carbon atoms such asdiethyl thiourea or monoethyl thiourea; saturated or unsaturated cyclichydrocarbons mono- or disubstituted thioureas such as naphthyl thiourea,diphenyl thiourea, cyclohexyl thiourea and the like, where the cyclichydrocarbons has up to about ten carbon atoms; the disulfide ofthiourea; thio-imidol (the reaction product of thiourea and sodiumhydroxide); thiocarbamic acid esters (the reaction products of thioureaand an alcohol comprising ROH where R is a saturated or unsaturatedaliphatic or cyclic group having up to about ten carbon atoms) theoligomers of thiourea and formaldehyde, e.g. monomethylol, dimethylol,and trimethylol thioureas; S-alkyl pseudo thioureas (manufactured by thereaction of thiourea with an iodo lower alkane such as iodo methanewhere the lower alkyl group contains up to about five carbon atoms);thiocarbonic acid esters of thiourea and R⁵ OH, (R⁵ as defined above)especially where R⁵ is lower alkyl; thioureadioxide (akaformamidinesulfinic acid [1758-73-2, C.A. Reg. No.]); the reactionproduct of a saturated or unsaturated aliphatic or cyclic organic acidhaving up to about 12 carbon atoms and especially the lower aliphaticmonocarboxylic acid reaction products with thiourea e.g. acylthioureas,and the mineral acid salts of thiourea e.g. thiourea mono- ordi-sulfate.

Similarly, the urea compound comprises either urea or the varioussubstituted ureas or urea reaction products such as biuret; monoalkyl ordialkyl urea, where the alkyl group comprises a lower alkyl group havingup to about four carbon atoms such as diethyl urea or monoethyl urea;saturated or unsaturated cyclic hydrocarbon mono- or disubstituted ureaswhere the cyclic hydrocarbon has up to ten carbon atoms, such asnaphthyl urea, diphenyl urea, cyclohexyl urea and the like; alkoxyethers of iso-urea especially lower alkoxy ethers of iso-urea where thelower alkyl group contains up to about five carbon atoms, these productsbeing manufactured by the reaction of a lower alkanol with cyanamidehydrochloride; acid derivatives of urea in which the hydrogen atom ofurea is substituted by an acyl group, these compounds sometimes beingreferred to as ureides obtained by the reaction of urea with amonocarboxylic aliphatic saturated or unsaturated organic acid having toabout 20 carbon atoms and especially those acids having up to about fourcarbon atoms; the mineral acid salts of urea e.g. urea mono- ordisulfate; triuret; cyanuric acid (a trimer of urea); ammelide (a trimerof urea); imidol; carbamic acid esters of urea and R⁵ OH, (where R⁵ isdefined above) especially alkyl carbamates made by the reaction productof an organic alchohol with urea especially an alkanol such as a loweralkanol containing up to about four carbon atoms; monomethylol urea,dimethylol urea, trimethylol urea and other oligomers of urea andformaldehyde. The various substituted ureas and their use in immersioncoatings is further disclosed by Ceresa, et al., in U.S. Pat. No.2,891,871 which is incorporated herein by reference.

Other urea and thiourea compounds that may be employed comprise:

    ______________________________________                                                urea nitrate;                                                                 urea oxalate;                                                                 urea phosphate;                                                               urea sulfate;                                                                 1-propylurea;                                                                 2-selenourea;                                                                 2-tellurourea;                                                                1(sulfonamylphenyl)urea;                                                      1,1,3,3-tetraethylurea;                                                       1,1,3,3-tetramethylurea;                                                      1,1,3,3-tetramethyl-2-thiourea;                                               1,1,3,3-tetraphenylurea;                                                      1,1,3,3-tetraphenyl-2-thiourea;                                               2-thio-1(2-tolyl)urea;                                                        2-thio-1(3-tolyl)urea;                                                        2-thio-1(4-tolyl)urea;                                                        2-thio-1,1,3-trimethylurea;                                                   2-thio-1,1,3-triphenylurea;                                                   1(2-tolyl)urea;                                                               1(3-tolyl)urea;                                                               1(4-tolyl)urea;                                                               1,1,3-trimethylurea;                                                  ______________________________________                                    

Some especially preferred thiourea compounds in addition to thioureathat may be employed according to the invention comprise:

    ______________________________________                                        phenylthiourea;                                                               naphthylthiourea;                                                             thiourea disulfide;                                                           oligomers of thiourea and formaldehyde;                                       N--allylthiourea;                                                             N--mono-p-tolylthiourea (and the equivalents thereof                          disclosed in U.S. Pat. No. 4,502,927, incorporated herein by                  reference);                                                                   N--alkylthioureas such as methylthiourea (and the equivalents                 thereof disclosed in West German Patent 3,110,478,                            incorporated herein by reference);                                            monophenylthiourea;                                                           metaphenylenedithiourea;                                                      N,N'--ethylenethiourea;                                                       N,N'--dibutyneylthiourea;                                                     N,N'--dibutenylthiourea;                                                      trifluoro acetylthiourea;                                                     isothiourea-S--propionic acid;                                                amino substituted thioureas such as thio semicarbazide and                    1-phenylthiosemicarbazide;                                                    1-aryl-thioureas where the aryl group is phenyl, benzyl or                    naphthyl.                                                                     ______________________________________                                    

When substituted urea and thiourea compounds are employed in lieu ofurea and thiourea it is preferred that each contain the samesubstituent. By eay of example, if diethylthiourea is used in thecomposition in lieu of thiourea, diethylurea would be substituted forurea and so forth.

The tin salt of the composition preferably comprises a stannous salt.Although stannous salts of an inorganic (mineral) acid or organic acidmay be used according to the invention (e.g. stannous formate, stannousacetate and the like) the tin salt of the present invention may comprisea stannous salt of a mineral acid such as the sulfur, phosphorous, andhalogen acids, especially the sulfur acids such as sulfuric acid orsulfamic acid. Alkali metal stannates may also be used such as sodium orpotassium stannate and the art known equivalents thereof. In oneembodiment of the invention stannous sulfate, stannous sulfamate orstannous acetate is used as the tin salt. Where tin lead coatings aredeposited, lead acetate may be used as the lead salt.

The acids that are employed according to the invention may be organicacids or inorganic acids (mineral acids) based on sulfur, phosphorous,or the halogens, the sulfur based mineral acids being preferred such assulfuric acid or sulfamic acid. Some of the organic acids that may beemployed according to the invention comprise monocarboxylic ordicarboxylic acids having up to about six carbon atoms such as formicacid, acetic acid, malic acid, maleic acid, and the like.

It is preferred, if possible not to use halogen acids or halogen saltssince halide residues will be produced in the metal coating deposited,these salts interfering with the electrical properties of the metal andmay also act as corrosive materials in the coating.

The chelating agents that may be employed generally comprise the variousclasses of chelating agents and specific compounds disclosed inKirk-Othmer, Encyclopedia of Chemical Technology, Third Edition volume5, pages 339-368 incorporated herein by reference. Chelating agents thatare especially preferred comprise the aminocarboxylic acids and thehydroxycarboxylic acids. Some specific aminocarboxylic acids that may beemployed in this respect comprise ethylenediaminetetraacetic acid,hydroxyethylethylenediaminetriacetic acid, nitrilotriacetic acid,N-dihydroxyethylglycine, and ethylenebis(hydroxyphenylglycine). Hydroxycarboxylic acids that may be employed comprise tartaric acid, citricacid, gluconic acid and 5-sulfosalicylic acid.

The various reducing agents that may be employed according to thepresent invention are well known in the art and generally compriseorganic aldehydes whether saturated or unsaturated, aliphatic or cyclic,having up to about ten carbon atoms. Lower alkyl aldehydes having up toabout six carbon atoms may be employed in this respect such asformaldehyde, acetaldehyde, propionaldehyde, butyraldehyde, and thelike. Especially preferred aldehydes comprise hydroxy aliphaticaldehydes such as glyceral-dehyde; erythrose; threose; arabinose and thevarious position isomers thereof and glucose and the various positionisomers thereof. Glucose has been found to act to prevent oxidation ofthe metal salts to a higher oxidation state (e.g., SnII to SnIV) butalso as a chelating agent and is especially useful for these reasons.

The surfactants that may be employed comprise any non-ionic, anionic,cationic or amphoteric surfactant such as those listed in Kirk-Othmer,Encyclopedia of Chemical Technology, Third Edition, Volume 22, pp.332-387 which is incorporated herein by reference. The non-ionicsurfactants are especially preferred.

What is claimed is:
 1. A process for improving the adhesion properties of a metal layer of a circuit board to a non-conductive surface of a circuit board comprising coating said layer with an immersion metal composition, said immersion metal composition comprising:(a) a thiourea compound (b) a urea compound (c) a metal saltsaid metal salt being selected from the group of salts based on the Group IVA metals including tin, Group VIII metals, Group IB metals, Group IIB metals and Group IIIA metals, said urea compound and thiourea compound each being present in an amount sufficient to enable said composition to function as an immersion composition.
 2. The process of claim 1 where said thiourea compound and said urea compound comprise a thiourea nucleus and a urea nucleus with the same substituents on each of said nuclei.
 3. The process of claim 1 where said thiourea compound comprises thiourea and said urea compound comprises urea.
 4. The process of claim 1 further comprising a compound selected from a member of the group consisting of a Group VIII metal salt, a Group IVA metal salt with the exception of tin, a Group IIB metal salt or a Group IB metal salt or mixtures thereof.
 5. The process of claim 1 where said composition further comprises a salt of an element selected from a member of the group consisting of iron, cobalt, or nickel or mixtures thereof.
 6. The process of claim 5 where said salt comprises a nickel salt.
 7. The process of claim 1 further comprising a salt of lead.
 8. The process of claims 4 or 5 further comprising a surfactant.
 9. The process of claims 4 or 5 further comprising polyoxyethylene nonyl phenol surfactant.
 10. The process of claim 1 where the components of said composition are present in the following amounts on a molar basis:(a) about 10 to about 125 parts of a thiourea compound (b) about 10 to about 125 parts of a urea compound (c) about 1 to about 15 parts of a tin salt.
 11. The process of claim 10 further comprising the following components on a molar basis:(d) about 5 to about 40 parts of a chelating agent (e) about 5 to about 110 parts of a reducing agent (f) about 1 to about 30 parts of an acid.
 12. The process of claim 10 further comprising a compound selected from the group consisting of a Group VIII metal salt, a Group IVA metal salt with the exception of tin, a Group IIB metal salt or a Group IB metal salt or mixtures thereof in an amount from about 0.2 parts to about 55 parts on a molar basis.
 13. A process for improving the adhesion properties of a metal layer of a circuit board to a non-conductive surface of a circuit board comprising coating said layer with an immersion metal composition, said immersion metal composition comprising:(a) a thiourea compound (b) a urea compound (c) a metal salt (d) a chelating agent (e) a reducing agent (f) an acidsaid metal salt being selected from the group of salts based on the Group IVA metals including tin, Group VIII metals, Group IB metals, Group IIB metals and Group IIIA metals, said urea compound and thiourea compound each being present in an amount sufficient to enable said composition to function as an immersion composition.
 14. The process of claim 1 or 13 where said tin salt comprises a stannous salt.
 15. The process of claim 13 where said reducing agent comprises an aldehyde reducing agent.
 16. The process of claim 13 where said acid comprises a mineral acid and said tin salt comprises a stannous salt of a mineral acid.
 17. The process of claims 1 or 13 or 4 or 5 where said metal layer comprises copper.
 18. The process of claims 1 or 13 or 4 or 5 where said printed circuit board comprises a multilayer board. 